1. Field of the Invention
The present invention relates to a production method of electronic parts having a step of working a substrate having disposed thereon a metal thin film conductor and also to an apparatus for producing water used in the above-described step.
2. Description of the Related Arts
Hitherto, in the case of working semiconductor substrates, such as, for example, washing, cutting, polishing, and a surface treatment, water is used. However, when water is sprayed onto a substrate at working, there is a problem that a static electricity is generated mainly by the friction between the water and the substrate. That is, in the case of working a substrate having formed thereon electrodes composed of a metal thin-film conductor, it sometime happens that the electrodes are destroyed by the generated static electricity. Recently, because electrodes are fined with small-sizing of electronic parts and increasing of high integration, the possibility of the destruction of the electrodes is more increased. To prevent the occurrence of the destruction of the electrodes by a static electricity, it is proposed to use an electrically conductive liquid such as a carbonic acid gas-intermixed water, etc., in the case of washing an Si wafer with high-pressure water in JP-A58-7831.
On the other hand, it is known that when in the case of working electronic parts using water, the amount of oxygen dissolved in the water is large, the electronic parts are corroded and oxidized. To prevent the occurrence of such a corrosion and oxidation, it is proposed to use a deoxidation water obtained by vacuum deairing via a deoxidation membrane for washing in JP-A-4-40270. The concentration of oxygen dissolved in the deoxidation water is 1 ppm or lower. Also, in JP-A-55-153332, a method of using water which was at least once boiled in the case of washing gallium arsenide semiconductor devices is proposed. The proposition is to prevent the oxidation of a gallium arsenide substrate.
The above-described propositions are all for the case of using semiconductor substrates but recently, electronic parts using piezoelectric substrates such as a surface acoustic wave (SAW) element, etc., have been increased. In the case of producing these electric parts, water is used at washing or cutting. However, because in a substrate having a pyroelectric property such as a piezoelectric substrate (for example, LiNbO3 and LiTaO3), an electrostatic charge generates even by heat and stress, the amount of electrostatic charge is greatly increased as compared with a semiconductor substrate. Also, in the electronic parts using piezoelectric substrates, in for example, a surface acoustic wave element, the electrode is thin and fine. In such circumstances, in the electronic parts using the piezoelectric substrates, particularly, the surface acoustic element, there is a problem that even when the water having the controlled electric conductivity and dissolved amount of oxygen same as the case of semiconductor substrates is used, the electrostatic destruction of the electrodes cannot be avoided.
Furthermore, in the surface acoustic wave element, to improve the electric power resistance, it is proposed to use two or more kinds of metals (for example, Al and Cu) for the electrodes but because in such an electrode, a local cell is formed by the potential difference between the two or more kinds of metals, the influence of the local cell is increased by the influenced with dissolved oxygen, the corrosion of the electrode is liable to proceed. Also, in an electronic part wherein an metal thin-film conductor is formed on a semiconductor substrate, a protective film made up of SiO2, etc., is further formed on the metal thin-film conductor and in the case of washing or in the case of obtaining plural elements by cutting the substrate, the corrosion of the metal thin-film conductor is hard to occur because the metal thin-film conductor is in the state of being protected with the protective film. On the other hand, in the surface acoustic wave element, a piezoelectric film is sometimes formed on a metal thin-film conductor but because the metal thin-film conductor is frequently exposed, the resistance to the corrosion is weak.
Then, by illustrating the production of a surface acoustic wave element having a metal thin-film electrode containing Al and Cu, the troubles occurring in the case of working using water are practically explained.
The troubles occurring at working using water are largely the following 1) to 3).
1) Electrode destruction by static electricity (static destruction).
2) Corrosion of electrodes occurring because the pH of water is outside a proper range.
3) Pitting caused by the action of dissolved oxygen in water as an oxidizing agent.
The static destruction of 1) occurs when the specific resistance of water becomes large. Hitherto, the specific resistance of water is lowered by dissolving a carbonic acid gas in pure water but because in the carbonic acid gas dissolving apparatus described in JP-A-58-7831, a carbonic acid gas is intermixed in a bath which is not open to the air, the carbonic acid gas is reluctant to be dissolved in pure water and hence it is difficult to lower the specific resistance of the water.
The corrosion of the electrode of 2) is caused by the dissolution of Al occurring when the pH of water is outside the pH range (generally, from 4 to 8) at which, Al can stably exist in the water.
The pitting of 3) occurs in the pH range (from 4 to 8) at which Al can stably exist in the water. In an Alxe2x80x94Cu electrode used for improving the electric power resistance in the surface acoustic wave element, a passive state (Al2O3) film exists on the surface of the electrode but when a defect exists in the passive state film, a local cell action occurs between Al and CuAl2 existing in the electrode, whereby Al is dissolved to cause pitting. In the course, dissolved oxygen becomes one of the factors of proceeding the occurrence of pitting. In JP-A-58-7831, the specific resistance of water is lowered by intermixing a carbonic acid gas but because in the carbonic acid gas dissolving apparatus described in the above-described patent publication, the carbonic acid gas is intermixed in a bath which is not opened in the air, the amount of dissolved oxygen in the water cannot be restrained. Thus, it is difficult to use the water produced by the apparatus described in the above-described patent publication for the production of surface acoustic wave elements.
The present invention has been made under these circumstances. An object of the present invention is to restrain the generations of the static electricity and the oxidation caused by the water used for working in the case of working substrates in the production step of electronic parts.
It has now been found that the above-described object can be attained by the invention shown by (1) to (7) described below.
(1) A production method of an electronic part comprising a step of working a substrate having formed thereon a metal thin-film conductor using water, wherein said water satisfies the following relations;
3.5xe2x89xa6pHxe2x89xa66.5
DOxe2x89xa60.5 pH2xe2x88x926.5 pH+22
[wherein; DO represents the dissolved oxygen amount (unit ppm)].
(2) A production method of an electronic part of (1) wherein the metal thin-film conductor contains at least two kinds of metals including Al.
(3) A production method of an electronic part of (1) wherein the substrate has a pyroelectric property.
(4) A water treatment apparatus comprising a mixing bath having formed an open portion, wherein the mixing bath is constructed such that a carbonic acid gas is intermixed in water to be treated by being bubbled and dissolved oxygen excluded from the water to be treated with the intermixed carbonic acid gas is discharged from the open portion.
(5) A water treatment apparatus of (4) wherein the apparatus has a water storage tank, the mixing bath has open portions at the upper portion and the lower portion respectively, dissolved oxygen excluded from the water to be treated is discharged from the upper open portion, and the carbonic acid gas-intermixed water to be treated is supplied to the water storage tank from the lower open portion.
(6) A water treatment apparatus of (4) wherein a carbonic acid gas is supplied to the water to be treated in the mixing bath though a porous filter.
(7) A water treatment apparatus of (4) wherein the water used in the production method of an electronic part of (1) is produced.